Location systems utilize one or more positioning mechanisms in order to determine the location of a terminal device, such as a mobile station, a user equipment or any other kind of radio terminal. Positioning a target terminal device involves signal measurements and a location estimate computation based on the measured signals. In general, a location or position estimate provides the geographic location of a mobile station and/or a valid mobile equipment, expressed in latitude and longitude data. The location estimate can be represented in a predetermined universal format.
Positioning mechanisms for location systems (LCS) in a GSM (Global System for Mobile communication) cellular system may be based on an uplink time of arrival (TOA) mechanism, Observed Time Difference (OTD) mechanisms (e.g. OTDOA or Enhanced OTD (E-OTD)), a Global Positioning System (GPS) assisted mechanism, cell identity (CI) based mechanisms, HLR (Home Location Register) or VLR (Visitor Location Register) queries, or any combination thereof. As a fall-back procedure, a Timing Advance (TA) parameter can be used to assist all above positioning mechanisms, except HLR/VLR based positioning. The TA value is usually known for the serving base transceiver station (BTS) to obtain TA values in case the concerned mobile station is in an idle mode. A special call not noticed by the user or subscriber of the mobile station is set up by using a paging procedure, e.g. not actually a call, and the cell identity (CI) of the serving cell and the TA is returned in response to this call.
In GSM, according to the uplink TOA positioning method, the time of arrival (TOA) of a known signal sent from the mobile station and received at three or more measuring units is measured. The known signal is an access burst generated by having the mobile station perform an asynchronous handover. The method requires an additional measurement unit hardware, i.e. a location measurement unit (LMU), in the network at the geographical vicinity of the mobile station to be positioned to accurately measure the TOA of the bursts. Since the geographical coordinates of the measurement units are known, the position of the mobile station can be calculated at a central location center via hyperbolic triangulation.
Furthermore, the E-OTD method is based on measurements in the mobile station of the enhanced observed time difference of arrival of bursts of nearby pairs of BTSs. To obtain an accurate triangulation, E-OTD measurements are needed for at least three distinct pairs of geographically dispersed BTSs. Based on the measured E-OTD values, the location of the mobile station can be calculated either in the network or in the mobile station itself, if all the needed information is available in the mobile station.
The GPS method refers to any of several variants that make use of GPS signals or additional signals derived from the GPS signals in order to calculate the position of the mobile station.
The location system is logically implemented in a cellular network through the addition of a network node, the Mobile Location Center (MLC). In particular, a Gate-way Mobile Location Center (GMLC) is provided, which is the first node which an external client accesses in the cellular network. The GMLC requests routing information from the Home Location Register (HLR), performs registration authorization and sends positioning request to and receives final location estimates from the network. Furthermore, a Serving Mobile Location Center (SMLC) is provided which manages the overall coordination and scheduling of resources required to perform positioning or location of a mobile or wireless terminal device. It also calculates the final location estimate and accuracy. In one cellular network, there may be more than one SMLC and GMLC.
A so-called NSS based SMLC supports positioning of a target mobile station via signaling to the visited Mobile Switching Center (MSC), while a so-called BSS based SMLC supports positioning via signaling to the Base Station Controller (BSC) serving the target mobile station. Both types of SMLC may support an interface to enable access to information owned by another SMLC.
The SMLC controls a number of LMUs for the purpose of obtaining radio interface measurements to locate or help locate mobile station subscribers in the area that it serves. The signaling between an NSS based SMLC and an LMU is transferred via the MSC serving the LMU, while the signaling between a BSS based SMLC and an LMU is transferred via the BSC that serves or controls the LMU.
The SMLC and GMLC functionality may be combined in the same physical node, combined in existing physical nodes, or reside in different nodes of the cellular network.
A more detailed description of the known location systems is disclosed in the GSM specification 03.71.
Document EP 0 905 992 A1 discloses a position location procedure where location requests are delivered to mobile terminals. A decision unit at the mobile terminals is arranged to measure the quality level of received radio signals and to switch to a special position location function when the measured quality level falls below a set level. Thereby, the position location procedure depends on the receiving quality of the radio signals and a high probability of success can be achieved
In the above conventional location systems, each location request generates a considerable amount of load to signaling, location calculation and measurement functions of the network. Therefore, enough signaling capacity and centralized calculation and/or measurement capacity has to be provided in the network for LCS purposes. However, current LCS architectures are no longer able to cope with the desired amount of location requests per second as defined by operator requirements.